1. Field of the Invention
The present invention relates generally to delay lines and, more specifically, to high-speed integrated-optical switchable delay lines.
2. Description of Related Art
In communication and radar systems, antenna elements are often arranged in space and connected to a signal source so as to produce a directional radiation pattern. Such arrangements are referred to as phased arrays. Phased arrays offer the advantages of high directivity and controlled antenna radiation pattern steering without the use of moving mechanical mechanisms.
Phased array antenna systems function by varying the phase of the signal supplied to the array elements. The variation of phase permits steering of the radiation pattern. By time-delaying the signal sent to various antenna elements in the array, a phase difference between the signals driving the antenna elements is created. By using a number of selectable time delays the radiation pattern can be effectively steered. As the time delays for each element are switched, the direction of the radiation pattern changes. The switching speed is the rate at which the delay time is switched. These delays can be created using either microwave techniques or optical techniques. Optical techniques traditionally use fiber optic cable or integrated optical waveguides arranged in a configuration with optical switches to create selectable time delays.
The advantages of using optical techniques over microwave techniques include: smaller and lighter delay devices, wider bandwidth of operation eliminating radiation pattern squint and enabling narrow pulse operation on large antennas, lower signal loss through the delay network, and higher signal isolation than traditional transmission lines.
At present there exists a number of methods for creating switchable time delays in optical systems. These methods all have inherent disadvantages as noted below.
Switching optical energy into various length optical fibers of the same material can be used to create a delay. In this configuration there is one optical input and one optical output, with multiple fiber optic paths that may be selected for various propagation lengths. Assuming the speed of propagation is the same for all of the fibers used, the varying lengths of the fibers determine the time delay. It is however, in practice, very difficult to cut the optical fibers with the precision necessary to create consistent delays. If high switching speeds are to be used, this method requires the use of specialized optical switches, which are typically integrated onto a substrate and increase the insertion loss of the delay unit. Additionally, it is difficult to align the precision-cut optical fibers with the integrated optical switches. Misalignment can lead to further increases in insertion loss of the optical system.
Another method for creating a switchable optical delay unit is distributing an optical signal source among fibers of various length, each fiber having an optical detector. The difference in the lengths of the fibers results in time differences for the signals that are output by the delay unit. To physically realize this method, an optical detector is needed for each fiber that is used, thus adding size and cost to the delay unit. For a desired delay, the proper detector output is selected and the remaining detectors are ignored. The distribution of the signal among multiple fibers leads to a reduction in the signal level that is received at any given detector, thus degrading the signal to noise ratio at the detector. As in the first method, the fibers must be precision cut to the proper length, which is very difficult and expensive.
A third method for creating switchable time delays in an optical system takes advantage of dispersion in optical fibers. This method uses the known dispersion characteristics of a given optical fiber to create a time delay. That is, different wavelengths will propagate at known different speeds in the fiber. Rapid switching of the frequency of an optical source, typically a laser or infrared emitter, will create a delay in the system. The problem with this method is that rapid switching of the optical source frequency is difficult to implement in practice.
Another method for creating an optical delay as proposed by Sullivan et. al. (SPIE Vol. 1703(1992) pp.264-271) consists of optical waveguides in a serpentine arrangement integrated onto a substrate. In this arrangement, a delay segment consists of two continuous waveguides used to create a long and a short optical path between optical switches. The long optical path serves the function of the delay path and the short path serves as the non-delay or reference path. The optical switches are used to channel the optical energy into the desired optical path. By cascading multiple delay sections, various time delays can be achieved. In applications where cross-talk isolation is critical, the Sullivan method of creating a time delay is inadequate. Cascading multiple delay sections, as designed according to the Sullivan method, leads to degradation of device cross-talk isolation.
Therefore, there exists the need for a delay section that is small, inexpensive, lightweight, requires only one optical detector, and has high cross-talk isolation even when multiple delay sections are cascaded together.